In a transmitter to be used in mobile telecommunications, a power efficiency and a linearity in a transmitting function are evaluated as an index for indicating a performance of the device. In particular, the power efficiency and the linearity in the transmitting function are the most important indices for representing the performance of the device in a high frequency modulation transmitting apparatus such as a cell phone.
For an amplifier in a final stage of the high frequency modulation transmitting apparatus, an amplifier for carrying out a so-called AB class operation has widely been used. The AB class amplifier has a small distortion, that is, is excellent in the linearity. On the other hand, a power with a DC bias component is always consumed. For this reason, a power efficiency is reduced.
For a method of operating a power amplifier with a high efficiency, therefore, there has been designed a polar coordinate modulating method for changing and amplifying a drain voltage or a collector voltage (a source voltage) corresponding to an amplitude component of a baseband signal by using a saturation region of an input/output characteristic of a transistor. Examples of the device of this type include an output variable transmitter disclosed in Japanese Patent No. 3044057 (Patent Document 1).
FIG. 12 is a block diagram showing a structure of an output variable transmitter according to a related example. The output variable transmitter has such a structure as to include modulating input terminals 101 and 102, a carrier oscillator 104, a quadrature modulator 103 for quadraturely modulating outputs of the modulating input terminals 101 and 102 at an output frequency of the carrier oscillator 104, a high frequency power amplifier 105, a transmitting output terminal 106, an envelope generating circuit 107 for generating an envelope from the outputs of the modulating input terminals 101 and 102, a specifying signal input terminal 112, a multiple-valued DC signal generating circuit 108 for inputting a signal for setting an average output level of the power amplifier 105 which is sent from the specifying signal input terminal 112 and generating a DC signal corresponding to an input value thereof, a multiplying circuit 109 for multiplying an output envelope of the envelope generating circuit 107 by an output of the multi-valued DC signal generating circuit 108, a voltage control circuit 110 for controlling a drain voltage of the power amplifier 105 corresponding to an output of the multiplying circuit 109, and a power terminal 111.
The quadrature modulator 103 modulates a carrier supplied from the carrier oscillator 104 in response to an I signal input from the modulating input terminals 101 and 102 and a Q signal which is quadrature to the I signal. The envelope generating circuit 107 calculates an amplitude signal R of the I and Q signals. An output level specifying signal corresponding to a transmitting output level which is to be output to the transmitting output terminal 106 is input to the specifying signal input terminal 112. The multiple-valued DC signal generating circuit 108 generates a DC signal in accordance with the output level specifying signal sent from the specifying signal input terminal 112.
The multiplying circuit 109 multiplies an output of the envelope generating circuit 107 by that of the multiple-valued DC signal generating circuit 108. Consequently, a signal which is proportional to an envelope of a modulation wave is obtained from the output of the multiplying circuit 109. The voltage control circuit 110 changes a drain voltage Vo of the power amplifier 105 in response to the output of the multiplying circuit 109. As a result, the drain voltage of the power amplifier 105 is proportional to the envelope of the modulation wave. By using the structure of the polar coordinate modulating method, accordingly, the power amplifier 105 can carry out a linear amplification while maintaining a saturation state having a high efficiency.
In the output variable transmitter using the polar coordinate modulating method according to the related example shown in FIG. 12, however, a saturation output is always reduced corresponding to a peak factor to be a ratio of a peak power to an average power. As a result, there is a problem in that the efficiency is deteriorated.
As means for solving the problem, there has been designed a method of multiplying a scaling factor to reduce the peak of the modulation wave when an instantaneous value of an amplitude exceeds a certain value. Examples of an apparatus of this type include a transmission wave generating method and apparatus disclosed in Japanese Patent No. 3024515 (Patent Document 2).
The following (1) and (2) are equations for explaining an operation of the transmission wave generating method and apparatus according to the related example.I(t)=Rmax×Iin(t)/Rin(t)  (1)Q(t)=Rmax×Qin(t)/Rin(t)  (2)
In the equations, Iin(t) and Qin(t) represent a quadrature modulating input signal, Rmax represents an amplitude limit value, and Rin(t) represents an estimated amplitude value calculated from Iin(t) and Qin(t).
In the related example, the estimated amplitude value Rin(t) is compared with the amplitude limit value Rmax. If Rin(t) is smaller than Rmax, the input signals Iin(t) and Qin(t) are exactly used. If Rin(t) is equal to or greater than Rmax, a peak of a modulating signal is reduced in accordance with the equations (1) and (2).
In the transmission wave generating method and apparatus according to the related example, however, an amplitude limitation is carried out only when an instantaneous value of an amplitude is greater than the amplitude limit value (a hard clipping operation) so that a rapid change is generated in the vicinity of the amplitude limit value. As a result, there is a problem in that a nonlinear distortion is easily increased.
Patent Document 1: Japanese Patent No. 3044057 (Pages 1 to 20, FIG. 1)
Patent Document 2: Japanese Patent No. 3024515 (Page 3, Equation 3)